1. Field of the Invention
The present invention relates to an external amplifier, and more particularly, to an apparatus and method for removing noise that is generated in an external amplifier when the audio output mode of a digital receiver is changed.
2. Discussion of the Related Art
A digital receiver, such as a digital television (DTV) or set-top box (STB), has a Sony Philips Digital Interface (SPDIF) output for digital audio transmission, which is connected to an external amplifier so as to be reproduced.
When audio is reproduced in the external amplifier, it is possible to obtain sound of higher quality, more channels, more various sound field effects, higher power, etc. than output sound from the digital receiver. Therefore, the user can enjoy a more realistic digital broadcast.
At present, the digital receiver can receive an analog broadcast, as well as a digital broadcast. Particularly, the digital television may receive an SPDIF input from an external digital device, such as a digital versatile disc (DVD) player, decode it and then provide the decoded result again as an SPDIF output, or may receive an analog L/R signal from an external analog device, such as a video cassette recorder (VCR), and provide it as an SPDIF output of a pulse code modulation (PCM) mode.
The SPDIF output mode is generally contained in an audio menu of the digital receiver, and is selected to suit the user's situation or automatically changed by the digital receiver according to a given broadcast or an external input source.
Audio transmitted when the user views a digital broadcast through the digital receiver, is an Encoded audio stream. The Encoded audio stream can be defined by various standards, such as AC3, Advanced Audio Coding (AAC) and Moving Picture Experts Group (MPEG). At present, the audio standard is AC3 in Korean and North American digital broadcastings.
In this digital audio environment, the user can select a desired one of an Encoded mode or PCM mode as the SPDIF output mode. For example, provided that the user has an amplifier capable of decoding an SPDIF output of the Encoded mode, he/she will generally select the SPDIF output of the Encoded mode to enjoy multichannel, high-quality sound.
In contrast, provided that the user has an amplifier incapable of decoding an SPDIF output of the Encoded mode, or an amplifier capable of receiving audio of an L/R 2 channel and providing a specific sound field effect, he/she will select an SPDIF output of the PCM mode. Similarly, for a digital television that receives an input from an external digital device such as a DVD player, the user can also select the SPDIF output mode.
On the other hand, in the case where the user views an analog broadcast through the digital receiver or an analog video from an external analog device connected with the digital receiver, the digital receiver analog/digital (A/D)—converts an analog L/R signal from the external analog device and provides the resulting signal as an SPDIF output of the PCM mode. In this case, even though the user selects the Encoded mode as the SPDIF output mode, the digital receiver automatically provides the SPDIF output in the PCM mode.
Accordingly, the user can change the SPDIF output mode from the PCM mode to the Encoded mode in the audio menu while viewing a digital broadcast, or the change of the SPDIF output mode from the PCM mode to the Encoded mode can be made upon a channel change from an analog channel to a digital channel, a source change from an analog channel to an external digital device, a source change from an external analog device to a digital broadcast, and a source change from an external analog device to an external digital device.
For a distinction between the Encoded mode and PCM mode of the SPDIF output, SPDIF channel status information is generally used, as will hereinafter be described with reference to the annexed drawings.
FIG. 1 illustrates the SPDIF channel status information.
As shown in FIG. 1, the channel status information includes various information for proper decoding of the SPDIF output by an external amplifier, in which the first bit of byte 0 indicates whether the SPDIF output mode is the PCM mode or Encoded mode.
That is, the first bit indicates the PCM mode if it is 0 and the Encoded mode if it is 1. This bit is also referred to as an Audio/Non Audio bit.
Therefore, the external amplifier determines whether the SPDIF output inputted thereto is in the Encoded mode or PCM mode, by periodically checking the first bit of byte 0 of the channel status information, and sets a decoder IC therein for proper decoding of the SPDIF output according to the determination result.
On the other hand, in this process of determining whether the SPDIF output is in the PCM mode or Encoded mode and processing the SPDIF output according to the determination result, noise is generated in the external amplifier when the SPDIF output mode is changed from the PCM mode to the Encoded mode, as will hereinafter be described with reference to FIG. 2.
FIG. 2 illustrates a process of noise generation in a conventional external amplifier when the SPDIF output mode is changed from the PCM mode to the Encoded mode.
When the SPDIF output mode of the digital receiver is changed from the PCM mode to the Encoded mode, the SPDIF data is changed from a PCM bit stream to an Encoded bit stream and, at the same time, the Audio/Non Audio bit of the channel status information is changed from the PCM mode (‘0’) to the Encoded mode (‘1’).
However, the external amplifier cannot immediately recognize the change of the SPDIF output mode from the PCM mode to the Encoded mode. For this reason, the external amplifier outputs an SPDIF input with an Encoded bit stream as sound of the PCM mode until it recognizes the mode change, causing noise.
In other words, the external amplifier has mode change check times T1, T2 and T4 at intervals of typically 100 ms or 200 ms, as shown in FIG. 2. In the case where the mode change is made at a time T3 between the check times, noise is generated for a period from the time T3 to the next check time T4.
In this case, the noise may be serious and the speaker may be subject to damage resulting from an abrupt sound variation.